FIX-deficient plasma from HTI was prepared by immunodepletion, whereas FIX-deficient plasma from Diagnostic Stago and George King Bio-Medical was prepared from FIX-deficient individuals. times and thrombin generation to measure the efficacy of a PS antibody for increasing FIX activity in commercially obtained plasma and plasma from pediatric patients with HB. We included 11 pediatric patients who lacked additional comorbidities and VP3.15 coagulopathies. In vivo,we assessed thrombus formation in VP3.15 HB mice in the presence of the FIXa PS antibody. We found an accelerated rate of clotting in the presence of PS antibody. Similarly, the peak thrombin formed was significantly greater in the presence of the PS antibody, even in plasma from patients with severe HB. Furthermore, HB mice injected with PS antibody and FIX had a 4.5-fold higher accumulation of fibrin at the thrombus induction site compared with mice injected with FIX alone. Our findings imply that a PS antibody would be a valuable adjunct to increase the effectiveness of FIX alternative therapy in pediatric patients who have moderate, moderate, and severe HB. == Introduction == The control of blood coagulation is an intricate process that depends on a stringently regulated balance between procoagulant and anticoagulant proteins. Any significant aberration in this balance can cause either a bleeding disorder (hemophilia) or untimely clotting (thrombosis). In the United States, about 20 000 individuals have been diagnosed with hemophilia and other related coagulopathies, and 400 infants are born each year with this group of genetic disorders.1,2 Hemophilia B (HB) is an X-linked hereditary bleeding disorder that results from a deficiency of the procoagulant protein factor IX (FIX).1,2The severity of the HB bleeding phenotype is related to the amount of remaining functional FIX. Normal plasma levels of FIX are 50 to 150 U/dL. The FIX reference range for the pediatric population aged 1 to 12 months is usually 44.0 to 114.0 U/dL, and for the population aged 1 to 18 years, it is 60.0 to 116.0 U/dL (Sysmex CA-1500 Analyzer).3Severe HB involves spontaneous, life-threatening bleeding in which the FIX levels are <1 IU/dL, usually diagnosed at birth. FIX levels of 1 to 5 IU/dL represent PIK3CD moderate HB, and 5 to 40 IU/dL are classified as moderate HB. Moderately and mildly affected individuals may have excessive bleeding only from trauma and/or surgical procedures. Currently, there is no cure for HB. There are VP3.15 effective treatments, but they require lifelong infusions of expensive drugs. The standard of care for HB management is usually either primary prophylaxis or secondary prophylaxis. Primary prophylaxis refers to the early initiation of clotting factor replacement therapy4,5before the onset of the first joint bleed but no later than before a second joint bleed, ideally before the age of 3 years. Conversely, secondary prophylaxis denotes the introduction of clotting factor alternative therapy after 2 joint bleeds before any evidence of joint disease. However, when Roth et al6studied plasma-derived FIX and recombinant FIX (rFIX), they identified limitations with rFIX regarding its pharmacokinetics, namely, a 25% to 30% lower in vivo recovery in children aged <15 years.7Moreover, replacement therapy may cause viral infections from natural clotting factors, and, because of delays in treatment, replacement therapy may cause damage to joints, muscles, or other parts of the body. In addition, for severe HB, replacement therapies have been associated with the occurrence of FIX inhibitors at frequencies of 3% to 5%. A PedNet study in 20218of an unselected cohort of previously untreated patients with severe HB showed a cumulative inhibitor incidence of 10.2% at 500 exposure days. Thus, considering the limitations of the current replacement therapies, investigators need to devise a novel treatment approach for HB. Below, we present results that support a proof-of-concept for a new therapy for augmenting FIX-based treatment, namely, coadministration of anti-PS antibodies in a pediatric patient population. Protein S (PS), a vitamin Kdependent protein, is a key unfavorable regulator in the coagulation cascade.9Deficiencies in PS cause disorders that range from a heightened risk of thrombosis.